This invention relates to power tools for cutting threads on the inner surface of a hole previously made in or through a plate, and more particularly to such a power tool having positive feed and automatic retract of the thread cutting tool.
U.S. Pat. No. 4,612,998 discloses my prior invention relating to a right angle positive feed drill with an automatic retraction mode, and an idle mode that automatically follows after the drill bit has been retracted. An object of this invention is to adapt the mechanism for automatic retract and idle modes of a positive feed drill of my prior U.S. Pat. No. 4,612,998 (which, by this reference, is made a part hereof) to a positive feed tapper, thus providing a right angle tapper that will automatically cycle through positive feed tapping and retracting modes into an idle mode.
The positive feed drill disclosed in the aforesaid patent has a drive gear train for turning a spindle that is threaded into a feed gear driven by a second gear train coupled to the first (drive) gear train through side teeth on coupling gears of the two gear trains that turn on a common shaft. Only the coupling gear of the first (drive) train on that shaft is driven by a motor. The gear ratio of that first gear train is selected to be slightly less than the gear ratio of the second (feed) gear train so that the feed gear into which the spindle is threaded will turn slightly faster than the spindle driven by the first (drive) gear train to advance the spindle.
The first (coupling) gear of the feed gear train is supported by a collar affixed to the common shaft so that as that shaft is lifted, the coupling gear of the feed gear train disengages from the corresponding gear of the drive gear train. Once the spindle has been advanced a predetermined extent, a hydraulic piston is automatically actuated to lift that common shaft. The first (coupling) gear of the feed gear train is thus raised by the shaft sufficiently to disengage from the coupling gear on the common shaft of the drive gear train and to lock upper side teeth on stationary teeth affixed to the drill housing. As the motor continues to drive the spindle in the same direction through the first gear train, the spindle continues to turn in the feed gear at the end of the second gear train which is then locked. The spindle is thus automatically retracted.
Once the spindle has been retracted the problem is to disengage the upper side teeth of the gear in the feed gear train to unlock the feed gear train without re-engaging lower side teeth of the feed coupling gear with side teeth of the corresponding coupling gear on the common shaft of the first (drive) gear train. That problem is solved in the positive feed drill disclosed in the aforesaid patent by removing the hydraulic pressure on the piston that lifts the common shaft when the spindle has been advanced a predetermined extent, thereby to allow a spring between the first gear of the feed gear train and the housing to force that first gear down sufficiently to unlock the side teeth of that first gear.
In the meantime, as the common shaft was lifted to retract the spindle, a notch in a rod at the base of the common shaft (into which the common shaft is normally seated by the force of the aforesaid spring) is cleared by the base of the shaft to allow the spring to move the rod laterally with respect to the axis of the shaft That causes the notch in the rod to move from under the shaft. Thereafter, when hydraulic pressure is removed from the piston and the first (coupling) gear of the feed gear train is forced down away from the locking teeth by the coiled spring around the common shaft, the common shaft will move down sufficiently to disengage the locking teeth on the housing but not sufficiently for lower side teeth of the first gear in the feed gear train to re-engage the upper side teeth of the corresponding gear of the spindle drive gear train.
The positive feed drill having an automatic retract mode is thus placed in an idle mode until the notched rod is manually forced in against its spring sufficiently to position its notch under the shaft, at which time the shaft moves down under the force of the spring around the common shaft, and the lower side teeth of the first gear of the feed gear train re-engage the upper side teeth of the corresponding gear of the spindle drive gear train for another drilling cycle.